The present invention relates to a method and apparatus including a high power laser for drilling gas, oil or geothermal wells in geological formations, and for fracturing the pay zones of such wells to increase recovery of oil, gas or geothermal energy.
The use of down-hole lasers (i.e. those lowered into the bore hole in proximity to the surface being drilled) for drilling through and recovering minerals from subterranean formations has been proposed, for example, in U.S. Pat. Nos. 3,871,485 and 3,882,945 to Keenen, and U.S. Pat. No. 3,461,964 to Venghiattis. However, the systems disclosed in these patents are inherently limited and impractical because adequate power cannot be obtained from lasers small enough to fit down the hole.
It has also been proposed, in the U.S. Pat. Nos. 3,493,060 to Van Dyk and 3,693,718 to Stout, to use a laser at the surface to penetrate through a mineral bearing formation or to form a subterranean cavity at the bottom of a bore hole. In those systems, a laser beam is directed down the bore hole to the bottom, where it drills by vaporizing or melting the irradiated formation. The depths to which such systems can operate, however, are extremely limited since the vapors and debris produced by drilling mix with the air or other gaseous medium in the bore hole, scattering and dissipating most of the beam energy before it can reach the rock to be drilled. No satisfactory method has been suggested for maintaining the beam path adequately free of contaminants and debris produced by the drilling itself to insure that the necessary levels of energy continue to reach the drilling site.
Moreover, there are no provisions in the proposed systems for guiding the beam so as to obtain adequate energy for drilling at the hole bottom. Since a beam from a high power laser undergoes significant divergence due to a number of factors, including non-uniform heating of the medium in its path, thermal distortion of the laser medium and optics and diffraction, beam spreading imposes an additional significant limitation on the operating depths of such systems.
Still a further drawback of proposed laser drilling methods renders them inadequate for any except the shallowest holes because they fail to suggest any way of isolating the high pressure down-hole environment from atmospheric pressure at the surface, while at the same time allowing transmission of the laser beam through the pressure barrier.
Fracing is a method of increasing the permeability and thus the flow rate of oil or gas from the producing formation into the bore holes so that it may be removed from the well. The most common type of fracing in use during the past decade has been hydraulic fracing, involving pumping large volumes of viscous fluid down-hole at a rate fast enough to fracture the formation along planes dictated by its natural weaknesses. The hydraulic fracing fluid commonly contains particulate matter, called "proppants" carried by the fluid through the fracture areas to prop the newly made fractures open permanently after the fluid pressure subsides. Explosive fracing has also been used to some extent although significant problems have been encountered in achieving reliable detonation in the very narrow propagation diameters required for this purpose. While so-called "bore-shots", or explosions of concentrated volumes of explosives in the bore hole proper, increase permeability somewhat simply by increasing the amount of fresh surface area available, they are generally not as effective as formation fracturing, which seeks to increase the permeability of the formation throughout a larger volume, extending far back from the bore hole.
All fracing methods previously in use are incapable of creating new fissures in the formation except along planes defined by the fault structure of the formation itself. This natural fault structure often bears no relation to the specific planes and directions where fracing would be most effective. Accordingly, it would be a significant breakthrough to be able to achieve fracing along directions from the bore hole that could be specifically related to the geology of the individual formation.